1. Field of the Invention
The present invention relates to a high precision rear projection type display system having an improved contrast and horizontal directivity and a broadened vertical field of view.
2. Description of the Related Art
FIG. 1 shows the structure of a conventional rear projection type display with a single projection lens. In FIG. 1, reference numeral 1 represents a color CRT as an original image forming means, reference numeral 2 represents a projection lens means, and reference numeral 3 represents a transmissive screen means.
FIG. 2 shows the structure of a conventional transmissive screen. For the details of its operation principle, refer to JP-A-58-59436, U.S. Pat. No. 4,725,134, U.S. Pat. No. Re. 33,795 and JP-A-60-263932. All disclosures of each of the above publications is incorporated herein by reference. Reference numeral 4 represents a Fresnel sheet for transforming divergent incident light beams to parallel beams, and reference numeral 5 represents a lenticular sheet for diverging light beams in the horizontal direction.
FIG. 3A is an enlarged partial plan view in cross section of the lenticular sheet 5. Reference numeral 6 represents the lenticular lens surface formed on the light input side, reference numeral 7 represents an ambient light absorbing stripe, reference numeral 8 represents a light transmissive part, and reference numeral 4' (refer to FIG. 2) represents a lenticular lens surface for diverging light beams in the vertical direction. Reference numeral 9 represents light beams diverged in the range of about 40 degrees to the right and left sides, reference numeral 10 and 10' represent light beams which are to be diverged in the range of about 50 degrees or more to the right and left sides.
The above-described conventional technique has at least the following three problems.
(1) As illustrated in FIG. 3A, in order to improve an image contrast, the width T of the light transmissive part 8 is required to be as narrow as possible, about 40% or less of the array pitch W of the lenticular lens 6 (a T/W ratio is called a duty factor of light transmission). However, with this settings, the light beams 10, 10' which are to be diverged in the range of about 50 degrees or more to the right and left sides are intercepted by the black stripes 7 as shown in FIG. 3A. The conventional technique is therefore associated with a problem that an image cannot be observed in the range of 50 degrees or more to the right and left sides if a contrast is to be improved.
(2) The lenticular sheet 5 is generally made of transparent methacrylic resin by extrusion roller molding. In order to display a high precision image on a screen having a diagonal length of about 50 inches (1 m width, and 0.75 m height), the array pitch W of the lenticular lens 6 is required to be about 0.4 mm or less, and the thickness of the lenticular sheet 5 is required to be about 0.6 mm or less. Problems to be caused by a precision limit of extrusion roller molding will be explained with reference to FIGS. 4A and 4B. Reference numerals 5, 6, 7, and 8 in FIGS. 4A and 4B represent similar elements to those shown in FIG. 3A. In the examples shown in FIGS. 4A and 4B, the edge of the light transmissive part 8 at its light output surface is made round because of an insufficient molding precision. The thickness of the lenticular sheet 5 shown in FIG. 4A is too smaller than a designed value, whereas the thickness of the lenticular sheet 5 shown in FIG. 4B is too larger than the designed value. Light beams 14, 14' have insufficient diverging angles to the right and left sides because of the round edges, and light beams 15, 15' indicate a total reflection at the light output surface. It is therefore difficult for the conventional technique to manufacture a screen of high quality and high precision.
(3) The inventor has found another problem that a lenticular sheet having a thickness of about 0.6 mm or less tends to buckle down by its weight. This case is illustrated in the vertical cross sectional view of FIG. 3B. Reference numerals 60 and 60' represent a screen frame for supporting the screen with torque, and reference numeral 5 represents a buckled lenticular sheet.
Analysis results of an Euler's bucking formula are given below. ##EQU1## EQU t: thickness EQU E.apprxeq.200 kg/mm.sup.2 (Young's modulus) EQU 1.apprxeq.0.75 m (screen height) EQU F.sub.2 .apprxeq.0.5 m.sub.3 l (pressure at screen half height)(2) EQU m.sub.3 .apprxeq.1.2 g/cm.sup.3 (density) EQU .thrfore.F.sub.2 =0.45 g/mm.sup.2 EQU .thrfore.t/1&gt;0.00083 ##EQU2##
The formula (1) is a general Euler's bucking prevention formula, and the formula (2) is a self-weight pressure.
As seen from the above formulas, an occurrence of the self-weight bucking phenomenon is theoretically proved for a 50-inch screen having a lenticular sheet of 0.62 mm thick or less.
Another problem of the conventional technique is a limited contrast of about 160:1 or less because of flares at the connective portion between the projection lens means 2 and the original image forming means 1 even when room illuminating ambient light is completely shut (even when a room light is turned off). A contrast visually and psychologically detectable by human eyes is about 300:1. It has been therefore desired to further improve a contrast.
Still another problem of the conventional technique is that an image contrast is lowered by reflection of a room illuminating ambient light on the screen if the light is very bright.
Another example of the structure of a conventional rear projection type display is shown in FIG. 5. In FIG. 5, reference numeral 101 represents a screen, reference numeral 102 represents a projection lens, reference numeral 103 represents an original image forming surface, reference numeral 104 represents an original image forming means, reference numeral 105 represents an output amplifier, reference numeral 106 represents a pre-amplifier, and reference numeral 107 represents an image signal input terminal. If the display is a CRT, the original image forming surface 103 is a face plate of CRT, and if the display is a liquid crystal display, it is a liquid crystal panel.
An example of the structure of a conventional transmission type screen is shown in FIG. 6. In FIG. 6, reference numeral 108 represents a Fresnel sheet, reference numeral 109 represents a vertically light-diverging lenticular sheet, reference numeral 110 represents a horizontally light-diverging lenticular sheet, reference numeral 112 represents a vertically micro-light-diverging lenticular lens surface having a pitch of about 80 .mu.m, reference numeral 111 represents a macro-light-converging Fresnel lens surface having a pitch of about 100 .mu.m, reference numeral 113 represents a horizontally micro-light-diverging lenticular lens surface having a pitch of about 500 .mu.m, and reference numeral 114 represents a black stripe surface. FIG. 7 is an enlarged cross sectional view of one pitch of the sheet 10. The light propagation direction is indicated by a solid line with arrows. Reference numeral 114' represents a light transmissive part, and reference numeral 114" represents a black strip part. A light diverging particle layer is generally provided to one of the sheets 8, 9, and 10. For the detailed explanation of FIGS. 6 and 7, refer to the above-cited U.S. Pat. No. 4,725,134 proposed by the present inventor.
FIGS. 8 and 9 are graphs showing the horizontal and vertical directivities of the screen shown in FIG. 7. A proper visual range is generally defined to be the angular range with a relative brightness of 1/3 or higher. This angular range is called a diverging angle in this embodiment. With the conventional technique, as seen from FIGS. 8 and 9, the horizontal diverging angle is about in excess of 90 degrees and the vertical diverging angle is about 20 degrees, which angles are constant over the whole screen. The gain of this screen is about 6.
These screen characteristics are sufficient for a current television system using about 250 scan lines per one field, but insufficient for a computer display having 1000 or more scan lines. The reason for this will be clarified in the following.
FIG. 10 shows a vertical directivity and a proper visual range of a conventional screen. Reference numeral 115 represents an output side conjugate point of a conventional screen.
A center beam of light beams outputted from each point on the screen propagates toward this conjugate point 115. H is an effective height of the screen. A solid line with an arrow indicates the light propagating direction, the range at the inside of oblique lines is a proper visual range.
In the case of a screen of a general television system, it is known that the distance from the screen to each viewer, i.e., a proper visual range, is 3 H to 8 H. The proper visual range shown in FIG. 10 contains this range from 3 H to 8 H. Therefore, this range is suitable for a screen of a general television system.
In the case of a screen of a computer display having 1000 or more scan lines, however, a user monitors the screen at the position nearer to the screen than a general television. FIG. 11 illustrates this case. Reference numeral 116 indicates the distribution of eye positions. It is desired that a shortest distance is about 1 H. To meet this requirement, the conventional technique has intensified the vertically light-diverging lenticular lens 112 shown in FIG. 6 uniformly to broaden the vertical diverging angle shown in FIG. 9 about twice (about 40 degrees). However, this approach lowers the screen gain by about one half and the brightness at each point on the whole screen by about one half, posing a problem of difficulty in viewing an image on the screen due to the lowered brightness. If a light source power is doubled in order to compensate for the degraded brightness, a power consumption becomes great and it takes an additional cost for heat dissipation.